Core-shell structure silicon carbide coated carbon microsphere composite ceramic powder and preparation method thereof

Abstract

The invention relates to core-shell structure silicon carbide coated carbon microsphere composite ceramic powder and a preparation method thereof. The core-shell structure silicon carbide coated carbon microsphere composite ceramic powder comprises a carbon microsphere as an inner core and a silicon carbide shell layer formed on the surface of the inner core.

Description

Technical Field

[0001] The present invention relates to a core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder and a preparation method thereof, belonging to the field of ceramic powder preparation. Background Art

[0002] Silicon carbide ceramics have excellent physical and chemical properties such as high hardness, high thermal conductivity, high temperature resistance, corrosion resistance, neutron irradiation resistance, and wear resistance, and are widely used in fields such as heat exchange components, precision bearings, and the semiconductor industry. Due to the excellent properties of silicon carbide, it has gradually become a favorable candidate material for seals in fields such as aerospace and nuclear power in China in recent years. However, silicon carbide has problems of high wear rate and large friction coefficient under dry friction conditions, resulting in easy failure due to wear under dry friction conditions and thus causing huge losses.

[0003] In order to improve the service performance of silicon carbide materials under dry friction and complex working conditions, self-lubricating composites based on silicon carbide are usually prepared. At present, there have been many studies on silicon carbide-carbon composites, and common preparation methods include atmospheric pressure sintering, hot pressing sintering, spark plasma sintering, and reaction sintering, etc., but these preparation methods all have certain deficiencies. For example, in atmospheric pressure sintering, hot pressing sintering, and spark plasma sintering, the bonding strength between carbon and silicon carbide is weak, resulting in the detachment of the hard material silicon carbide during the friction process, thus leading to damage during friction. In reaction sintering, the molten silicon with high activity in liquid-phase silicon infiltration causes violent reactions during the silicon infiltration process, resulting in less carbon content in the prepared silicon carbide-carbon composite and being difficult to apply.

[0004] Therefore, in order to solve the above problems, it is necessary to form a uniform coating layer through an interface modification method. On the one hand, it can be used to enhance the combination of the two phases, and on the other hand, the uniform coating layer can hinder the violent reaction between Si and C during the reaction sintering process. Currently, common interface modification methods include precursor infiltration pyrolysis method, chemical vapor deposition method, ion spraying, and other methods. However, the methods of precursor infiltration pyrolysis and ion spraying have problems of uneven coating, while chemical vapor deposition equipment is expensive and the powder is difficult to disperse evenly, resulting in difficulty in actual application. Therefore, it is urgent to prepare an MCMB@SiC powder with low cost, simple production process, and uniform core-shell structure. Summary of the Invention

[0005] Based on the deficiencies of the existing technology, the present invention aims to provide a preparation method for a core-shell structured silicon carbide-mesophase carbon microsphere composite powder to improve the bonding strength between the two phases and better protect the carbon source.

[0006] On the one hand, the present invention provides a core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder, including: a carbon microsphere as the inner core, and a silicon carbide shell layer formed on the surface of the inner core.

[0007] Preferably, the phase composition of the silicon carbide shell layer is α-SiC phase.

[0008] Preferably, the particle size of the carbon microspheres is 5-40 μm; the thickness of the silicon carbide shell layer is 0.1-2 μm.

[0009] Preferably, the total particle size of the core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder is 5.1-42 μm.

[0010] On the other hand, the present invention provides a method for preparing a core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder, comprising: (1) Using Si powder as a raw material, using mesophase carbon microspheres as a matrix, and using molten salt as a reaction medium, after mixing, a mixed powder is obtained; (2) Reacting the mixed powder in a protective atmosphere at 1200°C to 1400°C for 3 to 5 hours, and then washing and drying to obtain the core-shell powder of the core-shell structured silicon carbide-coated mesophase carbon microspheres.

[0011] In the present invention, adding salt as a medium in the preparation of the raw materials of the silicon carbide and mesophase carbon microsphere core-shell powder forms molten salt during the high-temperature firing process, providing a liquid phase environment for the dissolution of silicon, enhancing the reaction activity of silicon and carbon, and reducing the reaction temperature of silicon and carbon.

[0012] Preferably, in step (1): the particle size range of the Si powder is 1-3 μm; the molar ratio of the silicon powder to the mesophase carbon microspheres is 1:(4-7); The molten salt medium is at least two of a mixture of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate, and sodium fluoride; the mass ratio of the total mass of the silicon powder and the mesophase carbon microspheres to the mass of the molten salt medium is 1:(3-6). Among them, the test effect is not good when using KCl alone, and due to the strong affinity of Si for F, it is also not possible to conduct experiments using NaF alone.

[0013] Preferably, the purity of the potassium chloride is ≥99%, the purity of the sodium chloride is ≥99%, the purity of the potassium nitrate is ≥99%, the purity of the sodium nitrate is ≥99%, and the purity of the sodium fluoride is ≥99%; when the molten salt medium is a mixture of one of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate and sodium fluoride, the mass ratio of one of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate and sodium fluoride to sodium fluoride in the molten salt medium is (8-10):1, preferably 9:1.

[0014] Preferably, in step (1): the mixing method is wet ball milling; the parameters of the wet ball milling include: selecting at least one of ethanol and acetone as the ball milling medium; selecting SiC balls as the grinding balls; the ball milling speed is 200 - 400 revolutions per minute; the ball milling time is 2 - 6 h.

[0015] Preferably, in step (2): the heating regime of the reaction is: heating to 800 °C at a heating rate of 4 °C / min and holding for 0.5 h, then heating to 1200 - 1300 °C at a heating rate of 2 °C / min and holding for 3 - 5 h, and then cooling with the furnace; The protective atmosphere is an inert atmosphere, preferably an argon atmosphere.

[0016] Preferably, in step (2): the washing is repeated washing; the repeated washing includes: first ultrasonic cleaning with an ultrasonic cleaner for 10 min, the number of ultrasonic times is 1 - 3 times; then using distilled water at 70 - 80 °C, washing repeatedly for 10 - 15 times; finally soaking and washing with hot distilled water at 70 - 80 °C, the soaking time is 10 min, and the number of times of soaking and washing with hot distilled water is 5 - 10 times; The drying temperature is 50 - 70 °C, and the time is 12 - 16 h.

[0017] Advantages of the present invention: The present invention provides a silicon carbide - mesophase carbon microsphere core - shell structure powder body and a preparation method thereof. The molten salt formed at high temperature reduces the reaction temperature, enabling the reaction to proceed at a lower temperature. And because the silicon and carbon are in - situ reactions under the molten salt method, the uniform thickness of its shell layer is controllable. And compared with other methods, such as chemical vapor deposition, ion spraying, etc., the process is simpler and the cost is relatively lower. Description of the drawings

[0018] Figure 1 X - ray diffraction pattern of the silicon carbide - coated mesophase carbon microsphere composite powder prepared in Example 1; Figure 2 Surface morphology diagram of the silicon carbide - coated mesophase carbon microsphere composite powder prepared in Example 1; Figure 3 Cross - sectional element distribution diagram of the silicon carbide - coated mesophase carbon microsphere composite powder prepared in Example 1; Figure 4 X - ray diffraction pattern of the silicon carbide - coated mesophase carbon microsphere composite powder prepared in Comparative Example 1; Figure 5 X - ray diffraction pattern of the silicon carbide - coated mesophase carbon microsphere composite powder prepared in Comparative Example 3. Detailed implementation manners

[0019] The present invention will be further described by the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, rather than limiting the present invention.

[0020] In the present disclosure, the silicon carbide-silicon carbide core-shell structure powder uses mesocarbon microbeads as the core structure and silicon carbide as the shell structure.

[0021] In the present invention, the raw materials for preparing the silicon carbide-mesocarbon microbead core-shell structure include a carbon source, a silicon source, and a molten salt medium; in the preparation of the silicon carbide-mesocarbon microbead core-shell powder, the thickness of the silicon carbide shell layer is 0.1-2 μm.

[0022] In the present invention, the silicon source is Si powder, and the carbon source is MCMB. The molten salt medium is selected from KCl and NaF, and the addition amount is 300-500 wt% of the total mass of the raw material powder.

[0023] The process of the present invention is simple, and the thickness of the shell layer is uniformly controllable. In-situ reaction occurs on the surface of the carbon microbeads by the molten salt method to generate silicon carbide, which can effectively enhance the bonding strength between silicon carbide and carbon microbeads (wherein, the in-situ reaction can enhance the bonding strength), and the prepared core-shell powder of silicon carbide-coated carbon microbeads can avoid the violent reaction of the carbon source and has a protective effect on the carbon source. The following exemplarily describes the preparation method of the silicon carbide-mesocarbon microbead core-shell powder in the present invention.

[0024] Mix the MCMB powder, Si powder, and molten salt medium evenly, put them into a ball mill, add a solvent to the ball mill for ball milling, and use SiC balls for ball milling to obtain a mixed slurry. The particle size of the MCMB powder is 5-40 μm, and the purity is ≥99%. The particle size of the Si powder is 1-3 μm, and the purity is ≥99.9%. The molten salt medium is KCl and NaF, and the addition amount is 300-500 wt% of the total mass of the raw material powder. The ratio of the powder to ethanol and SiC balls is controlled at 1:(1.0-1.2):(1-2), preferably 1:(1.1-1.2):1.

[0025] Dry the mixed slurry to obtain a mixed powder of MCMB-Si-molten salt medium;

[0026] Pour the MCMB-Si-molten salt mixed powder into an alumina crucible, and place the alumina crucible in a tubular furnace for heating reaction, and wait for it to cool. Among them, the temperature of the heating reaction is 1200-1300 °C, the holding time is 3 h, and the sintering atmosphere is an argon atmosphere.

[0027] Wash the reacted powder repeatedly to remove the molten salt medium, and finally dry it to obtain the core-shell powder of silicon carbide-mesocarbon microbeads.

[0028] In the present invention, a molten salt medium is added to the raw materials for preparing the silicon carbide-mesophase carbon microsphere core-shell powder. During the sintering process, the molten salt medium melts at high temperature to form a liquid salt, which enhances the reaction activity of silicon and carbon and reduces the reaction temperature between the two. During the sintering process, the dissolved silicon reacts with the carbon atoms on the surface of the mesophase carbon microspheres to form a silicon carbide layer that is uniform, continuous, and has a controllable thickness of silicon carbide.

[0029] The following further gives examples to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, rather than being limited to the specific values in the following examples.

[0030] Example 1 Weigh the reaction raw materials according to the molar ratio of Si powder (1 - 3 μm) to mesophase carbon microspheres (with a direct diameter of about 5 μm) = 1:4, weigh the raw material powder according to the mass ratio of the raw material powder to the molten salt medium (KCl:NaF = 9:1) = 1:3, and uniformly mix according to the mass ratio of the reaction raw material powder, absolute ethanol, and silicon carbide grinding balls = 1:1:2. Use a planetary ball mill to mill for 4 hours. After drying, sieve the powder, and place the mixed and dried powder in an alumina crucible. Under a protective atmosphere of argon, heat it to 800 °C at a heating rate of 4 °C / min and hold for 30 min, then heat it to 1200 °C at a heating rate of 2 °C / min and hold for 3 h, and then cool it to room temperature with the furnace. Subsequently, take out the crucible after the high-temperature reaction, perform ultrasonic dispersion in an ultrasonic cleaner, and then repeatedly wash it with distilled water at 70 °C for 30 times. Then place the sample in an oven at 60 °C for 14 h of drying. Obtain the silicon carbide-mesophase carbon microsphere core-shell structure composite powder. Use an X-ray diffractometer to perform XRD tests on the obtained powder, and the results are as Figure 1 shown. It can be determined that silicon carbide ceramics are formed. Use a field emission scanning electron microscope to characterize the obtained powder, and the powder morphology is as Figure 2 shown. It can be seen that the silicon carbide ceramic layer is continuous and complete, the coating effect is good and the thickness is uniform, and the diameter of the coated powder is about 40 μm. The cross-sectional view and element distribution of the powder are as Figure 3 shown. It can be seen that the shell layer thickness is about 0.3 μm, and the silicon element is uniformly distributed in the coating layer, indicating that the silicon carbide-coated carbon microsphere composite powder is formed.

[0031] Example 2 The process of this example is the same as that of Example 1, except that there are certain changes in the raw material ratios in the process parameters. The ratios between the raw materials are adjusted to a certain extent. The molar ratio of Si powder to mesophase carbon microspheres (with a diameter of about 15 μm) is 1:5, the mass ratio of raw material powder to molten salt medium = 1:4, and the reaction temperature is 1200 °C for 4 h of heat preservation. The same characterization tests as in Example 1 are carried out on the powder of this example, and the results are as follows: the powder diameter is about 15 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0032] Example 3 The process of this example is the same as that of Example 1, except that there are certain changes in the raw material ratios in the process parameters. The ratios between the raw materials are adjusted to a certain extent. The molar ratio of Si powder to mesophase carbon microspheres (with a diameter of about 25 μm) is 1:6, the mass ratio of raw material powder to molten salt medium = 1:5, and the reaction temperature is 1200 °C for 5 h of heat preservation. The same characterization tests as in Example 1 are carried out on the powder of this example, and the results are as follows: the powder diameter is 25 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0033] Example 4 The process of this Example 4 is the same as that of Example 1, except that there are certain changes in the raw material ratios in the process parameters. The ratios between the raw materials are adjusted to a certain extent. The molar ratio of Si powder to mesophase carbon microspheres (with a powder diameter of about 40 μm) is 1:4, the mass ratio of raw material powder to molten salt medium = 1:3, and the reaction temperature is 1300 °C for 3 h of heat preservation. The same characterization tests as in Example 1 are carried out on the powder of this example, and the results are as follows: the powder diameter is 41 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0034] Example 5 The process of this Example 5 is the same as that of Example 1, except that there are certain changes in the raw material ratios in the process parameters. The ratios between the raw materials are adjusted to a certain extent. The molar ratio of Si powder to mesophase carbon microspheres (with a powder diameter of about 30 μm) is 1:5, the mass ratio of raw material powder to molten salt medium = 1:4, and the reaction temperature is 1300 °C for 4 h of heat preservation. The same characterization tests as in Example 1 are carried out on the powder of this example, and the results are as follows: the powder diameter is 30 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1.7 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0035] Example 6 The process of Example 6 is the same as that of Example 1, except that there are certain changes in the raw material ratio among the process parameters. The ratio between the raw materials is adjusted to a certain extent. The molar ratio of Si powder to mesophase carbon microspheres (powder diameter is about 20 μm) is 1:6, the mass ratio of raw material powder to molten salt medium = 1:5, and the reaction temperature is 1300 °C with heat preservation for 5 h. The powders of Example are subjected to the same characterization tests as those of Example 1, and the results are as follows: the powder diameter is 20 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1.9 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0036] Example 7 The process of Example 7 is the same as that of Example 3, except that: the mass ratio of raw material powder to molten salt medium = 1:3. The results are as follows: the powder diameter is 8 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 0.5 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0037] Example 8 The process of Example 8 is the same as that of Example 3, except that: the mass ratio of raw material powder to molten salt medium = 1:4. The results are as follows: the powder diameter is 35 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1.2 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0038] Example 9 The process of Example 9 is the same as that of Example 3, except that: the molar ratio of Si powder to mesophase carbon microspheres (diameter is about 25 μm) is 1:4. The results are as follows: the powder diameter is 25 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 1.1 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0039] Example 10 The process of Example 10 is the same as that of Example 3, except that: the molar ratio of Si powder to mesophase carbon microspheres (diameter is about 25 μm) is 1:5. The results are as follows: the powder diameter is 34 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 0.9 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0040] Example 11 The process of Example 11 is the same as that of Example 3, except that: KCl:NaF = 8:1. The results are as follows: the powder diameter is 17 μm, and the microstructure is similar to that of Figure 2 and the shell thickness is about 0.8 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0041] Example 12 The process of Example 12 is the same as that of Example 3, except that: KCl:NaF = 10:1. The results are as follows: the powder diameter is 22 μm, and the microstructure is the same as that of Figure 2 the similar one; the shell thickness is about 1.2 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0042] Example 13 The process of Example 13 is the same as that of Example 3, except that: the mass ratio of raw material powder to molten salt medium = 1:6. The results are as follows: the powder diameter is 30 μm, and the microstructure is the same as that of Figure 2 the similar one; the shell thickness is about 1 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0043] Example 14 The process of Example 14 is the same as that of Example 3, except that: the molar ratio of Si powder to mesophase carbon microspheres (with a diameter of about 25 μm) is 1:7. The results are as follows: the microstructure is the same as that of Figure 2 the similar one; the shell thickness is about 1.3 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0044] Example 15 The process of Example 15 is the same as that of Example 3, except that: the reaction temperature is 1400 °C and the time is 5 h. The results are as follows: the powder diameter is 40 μm, and the microstructure is the same as that of Figure 2 the similar one; the shell thickness is about 1.6 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0045] Example 16 The process of Example 16 is the same as that of Example 3, except that: KCl:NaF = 6:1. The results are as follows: the powder diameter is 40 μm, and the microstructure is the same as that of Figure 2 the similar one; the shell thickness is about 1.2 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0046] Example 17 The process of Example 17 is the same as that of Example 3, except that: KCl:NaF = 4:1. The results are as follows: the powder diameter is 20 μm, the shell thickness is about 0.5 μm; the Si element is evenly distributed in the shell, and the shell composition is SiC.

[0047] Comparative Example 1 The process of Comparative Example 1 is the same as that of Example 3, except that: the mass ratio of raw material powder to molten salt medium = 1:2. The results are as follows: its XRD results are as Figure 4 shown, and there is a large amount of silicon residue after the reaction, indicating that the reaction is not complete.

[0048] Comparative Example 2 The process of Comparative Example 2 is the same as that of Example 3, except that the molar ratio of Si powder to mesocarbon microspheres (with a diameter of about 25 μm) is 1:3. The results are as follows: Its XRD results are the same as Figure 4 the results of [comparative object], and there is a large amount of silicon residue after the reaction, indicating that the reaction is not complete.

[0049] Comparative Example 3 The process of Comparative Example 3 is the same as that of Example 3, except that the reaction temperature is 1100 °C and the time is 5 h. The results are as follows: Its XRD results are as Figure 5 shown, and no obvious SiC peak is detected after the reaction, indicating that the reaction does not occur.

[0050] Comparative Example 4 The process of Comparative Example 4 is the same as that of Example 3, except that KCl:NaF = 2:1. The results are as follows: Its XRD results are the same as Figure 4 the results of [comparative object], and there is a large amount of silicon residue after the reaction, indicating that the reaction is not complete.

[0051] Comparative Example 5 The process of Comparative Example 5 is the same as that of Example 3, except that the molten salt medium is NaF. Since Si has a strong affinity for F, no SiC product can be prepared if it is all NaF.

[0052] Comparative Example 6 The process of Comparative Example 6 is the same as that of Example 3, except that the molten salt medium is KCl. The results are as follows: Its XRD results are the same as Figure 5 [comparative object], and no obvious SiC peak is detected after the reaction, indicating that the reaction does not occur.

[0053] Table 1 is as follows:

[0054] In summary, the present invention proposes a simple and efficient method for preparing silicon carbide-mesocarbon microsphere composite powder. The silicon carbide phase uniformly coats the surface of the carbon microspheres, and the shell layer of the composite powder is tightly combined with its carbon microspheres. Dry pressing and sintering densification of the obtained powder can effectively improve the problem of poor bonding force between SiC and C, play a role in protecting the carbon source during the silicon infiltration process, and can significantly improve the mechanical properties of the material and its application with a high mass fraction of C source.

Claims

1. A core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder, characterized in that, Comprising: A carbon microsphere as the core, and a silicon carbide shell layer formed on the surface of the core.

2. The core-shell structured silicon carbide-coated carbon microsphere ceramic powder according to claim 1, characterized in that, The phase composition of the silicon carbide shell layer is the α-SiC phase.

3. The core-shell structured silicon carbide-coated carbon microsphere ceramic powder according to claim 1 or 2, characterized in that, The particle size of the carbon microsphere is 5 - 40 μm; the thickness of the silicon carbide shell layer is 0.1 - 2 μm.

4. The core-shell structured silicon carbide-coated carbon microsphere composite powder according to any one of claims 1-3, characterized in that, The total particle size of the core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder is 5.1 - 42 μm.

5. A preparation method of the core-shell structured silicon carbide-coated carbon microsphere composite ceramic powder according to any one of claims 1-4, characterized in that, Comprising: (1) Using Si powder as the raw material, mesophase carbon microspheres as the matrix, and molten salt as the reaction medium. After mixing, a mixed powder is obtained. (2) Reacting the mixed powder in a protective atmosphere at 1200°C - 1400°C for 3 - 5 hours, followed by washing and drying to obtain the core-shell powder of silicon carbide-coated mesophase carbon microspheres with the core-shell structure.

6. The preparation method according to claim 5, characterized in that, In step (1): The particle size range of the Si powder is 1 - 3 μm; the molar ratio of the silicon powder to the mesophase carbon microspheres is 1:(4 - 7). The molten salt medium is at least two of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate, and sodium fluoride; the mass ratio of the total mass of the silicon powder and mesophase carbon microspheres to the mass of the molten salt medium is 1:(3 - 6).

7. The preparation method according to claim 5, characterized in that, The purity of the potassium chloride is ≥99%, the purity of the sodium chloride is ≥99%, the purity of the potassium nitrate is ≥99%, the purity of the sodium nitrate is ≥99%, and the purity of the sodium fluoride is ≥99%. When the molten salt medium is a mixture of one of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate and sodium fluoride, the mass ratio of one of potassium chloride, sodium chloride, potassium nitrate, sodium nitrate to sodium fluoride in the molten salt medium is (4 - 10):1, preferably (8 - 10):1, and most preferably 9:

1.

8. The preparation method according to claim 5, characterized in that, In step (1): The mixing method is wet ball milling; the parameters of the wet ball milling include: selecting at least one of ethanol and acetone as the ball milling medium; selecting SiC balls as the grinding balls; the ball milling speed is 200 - 400 revolutions per minute; the ball milling time is 2 - 6 h.

9. The preparation method according to claim 5, characterized in that, In step (2): The heating regime of the reaction is: heating to 800°C at a heating rate of 4°C / min and holding for 0.5 h, then heating to 1200 - 1300°C at a heating rate of 2°C / min and holding for 3 h - 5 h, and then cooling with the furnace. The protective atmosphere is an inert atmosphere, preferably an argon atmosphere.

10. The preparation method according to any one of claims 5-9, characterized in that, In step (2): The washing is repeated washing; the repeated washing includes: first ultrasonic cleaning with an ultrasonic cleaner for 10 min, the number of ultrasonic times is 1 - 3 times; then using distilled water at 70 - 80°C and washing repeatedly for 10 - 15 times; finally soaking and washing with hot distilled water at 70 - 80°C, the soaking time is 10 min, and the number of times of soaking and washing with hot distilled water is 5 - 10 times. The drying temperature is 50 - 70°C and the time is 12 - 16 h.

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